Environmental interests and an increasing demand for diesel fuel encourage fuel producers to employ more intensively available renewable sources. However, known processes, utilizing such sources tend to result in an increase in carbon dioxide emissions with generally known negative effects. In the manufacture of diesel fuel the main interest is focused on vegetable oils and animal fats comprising triglycerides of fatty acids. Long, straight and mostly saturated hydrocarbon chains of fatty acids correspond chemically to the hydrocarbons present in diesel fuels. However, the neat vegetable oils display inferior properties, particularly extreme viscosity, and thus their use in fuels is limited.
Conventional approaches for converting vegetable oils into fuels comprise transesterification, hydrogenation and cracking, among others. Triglycerides, which form the main component in vegetable oils, are converted into the corresponding esters by the transesterification reaction with an alcohol in the presence of catalysts. However, poor low-temperature properties of the products obtained limit their wider use in regions with colder climatic conditions. Schmidt, K., Gerpen J. V.: SAE paper 961086 teaches that the presence of oxygen in esters results in undesirable higher emissions of NOx, in comparison to conventional diesel fuels.
Thermal and catalytic cracking of bio-materials like vegetable oils and animal fats lead to a wide spectrum of products. U.S. Pat. No. 5,233,109 describes an example of such process using catalysts containing alumina and another component, such as silica or alumino-silicate. The reactions are generally unselective and less valuable products are formed as well. The unsaturated and aromatic hydrocarbons present in the liquid fraction make these products unattractive for the diesel pool.
U.S. Pat. No. 4,992,605 and U.S. Pat. No. 5,705,722 describe processes for the production of diesel fuel additives by conversion of bio-oils into saturated hydrocarbons under hydroprocessing conditions. The conversion of the carboxylic group into a methyl group requires relatively high hydrogen partial pressure of above 4.5 MPa. Hydrogen consumption is further increased due to eventual side reactions such as methanation and reverse water-gas shift reaction. The high hydrogen consumption limits the use of such processes, especially in refineries where the hydrogen balance is already almost negative because of complying with legislative requirements.
Undesired oxygen may be removed from fatty acids or esters by deoxygenation. The deoxygenation of bio-oils and fats to hydrocarbons, suitable as diesel fuel products, may be performed in the presence of catalysts under hydroprocessing conditions. During hydrodeoxygenation conditions oxogroups are hydrogenated and therefore this reaction requires rather high amounts of hydrogen. Additionally, hydrogen is consumed in side reactions as well.
Decarboxylation of fatty acids results in hydrocarbons with one carbon atom less than the original molecule. The feasibility of decarboxylation varies greatly with the type of carboxylic acid used as the starting material. Activated carboxylic acids containing electron-attracting substituents in the position alpha or beta with respect to the carboxylic group lose carbon dioxide spontaneously at slightly elevated temperatures. In this case, the RC—COOH bond is weakened by the electron shift along the carbon chain.
The majority of fatty acids are, however, not activated. The positive induction effect of the carbon chain evokes a high electron density in the position alpha with respect to the carboxylic group making thus the release of CO2 difficult. Although the decarboxylation of activated and non-activated carboxylic acids is thermodynamically comparable, the activation energy is significantly higher in the case of the latter one. Therefore drastic conditions or the presence of a catalyst are required to overcome the energetic barrier.
The fusion of alkaline salts of fatty acids with the corresponding hydroxides to hydrocarbons is known already from the 19th century. The reaction is highly unselective and ketones and cracking products, as well as undesired highly alkaline waste are formed with low conversion.
Further, there exist a number of decarboxylation reactions used mainly in organic synthesis. Most of them proceed via free radical mechanism.
U.S. Pat. No. 4,262,157 discloses a decarboxylation process utilizing diazacycloalkenes and Cu salts, wherein lauric acid reacts to form n-undecane with 51% yield at 320° C. Also decarboxylation of unsaturated acids to form hydrocarbons with one carbon less is described.
Indirect decarboxylation routes are also known, involving transformation of carboxylic acids into the corresponding halides, followed by their dehalogenation. Hunsdiecker's and Kochi's reactions are examples of such reactions and both reactions proceed via free radical mechanism.
Available alternative routes involve electrochemical and photo-catalytic decompositions. An example of electrochemical decomposition is the Kolbe electrolysis, wherein the reaction is started by anodic mono-electron oxidation leading to the formation of carboxylate radicals. Their subsequent decarboxylation results in probable formation of hydrocarbon radicals. Their dimerization or less often disproportionation leads to the termination of the free radical reaction. The electrolytic systems for the hydrocarbon synthesis usually comprise aqueous solvents, organic co-solvents, added salts and platinum electrodes. Under such conditions the reaction yields 50-90% of coupling hydrocarbon products. The main side products comprise 1-unsaturated hydrocarbons formed via disproportionation. A similar radical mechanism applies also for photo-catalytically initiated reactions.
Two step deoxygenation of oxygen-containing bio-oil compounds is described by Parmon et al: Catalysis Today 35 (1997) 153-162. The model compound, phenol, is in the first step treated with carbon monoxide over bimetallic alloy RhCu. The product, benzoic acid, consequently decarboxylates in the presence of PtPd or RuPd alloys in the second step
The complexity of the decarboxylation reactions listed above and/or the low yield and very often, also the hazardous materials applied in the reactions, are the main drawbacks of these approaches.
Decarboxylation of carboxylic acids to hydrocarbons by contacting carboxylic acids with heterogeneous catalysts was suggested by Maier, W. F. et al: Chemische Berichte (1982), 115(2), 808-12. They tested Ni/Al2O3 and Pd/SiO2 catalysts for decarboxylation of several carboxylic acids. During the reaction the vapors of the reactant passed through a catalytic bed together with hydrogen. Hexane represented the main product of the decarboxylation of the tested compound heptanoic acid. When nitrogen was used instead of hydrogen no decarboxylation was observed.
U.S. Pat. No. 4,554,397 discloses a process for the manufacture of linear olefins from saturated fatty acids or esters. The catalytic system consists of nickel and at least one metal selected from the group consisting of lead, tin and germanium. According to the examples, when other catalysts, such as Pd/C were used, low catalytic activity, cracking to saturated hydrocarbons or formation of ketones when Raney-Ni was used, were observed.
Decarboxylation, accompanied with hydrogenation of oxo-compound, is described in Laurent, E., Delmon, B.: Applied Catalysis, A: General (1994), 109(1), 77-96 and 97-115, wherein hydrodeoxygenation of biomass derived pyrolysis oils over sulphided CoMo/γ-Al2O3 and NiMo/γ-Al2O3 catalysts was studied. Di-ethyldecanedioate (DES) was used among others as a model compound and it was observed that the rates of formation of the decarboxylation product (nonane) and the hydrogenation product (decane) were comparable under hydrotreating conditions (260-300° C., 7 MPa, in hydrogen). NiMo/γ-Al2O3 showed slightly higher selectivity towards decarboxylation products in comparison to CoMo/γ-Al2O3 catalyst. The presence of hydrogen sulphide, in contrary to ammonia, also promoted the decarboxylation, particularly when NiMo catalysts were used.
A process for converting an ester-containing vegetable oil into hydrocarbons is disclosed in GB 1,524,781. The conversion to hydrocarbons is performed over a catalyst containing an admixture of silica-alumina with an oxide of a transition state metal of groups IIA, IIIA, IVA, VA, VIA, VIIA, or VIIIA of the periodic table at the reaction temperatures of 300-700° C. The products formed are reported to be free from oxygenated compounds (other than carbon dioxide and water). In accordance with the examples, extensive cracking is, however, observed.
Based on the above it can be seen that there exists an evident need for an industrially applicable catalytic method for the selective manufacture of hydrocarbons from renewable sources, utilising the decarboxylation reaction.